Abstract base class for input from a file in a Directory. A
random-access input stream. Used for all Lucene index input operations.

IndexInput may only be used from one thread, because it is not
thread safe (it keeps internal state like file position). To allow
multithreaded use, every IndexInput instance must be cloned before
used in another thread. Subclasses must therefore implement clone(),
returning a new IndexInput which operates on the same underlying
resource, but positioned independently. Lucene never closes cloned
IndexInputs, it will only do this on the original one.
The original instance must take care that cloned instances throw
AlreadyClosedException when the original one is closed.

length

toString

clone

Clones of a stream access the same data, and are positioned at the same
point as the stream they were cloned from.

Expert: Subclasses must ensure that clones may be positioned at
different points in the input from each other and from the stream they
were cloned from.

Warning: Lucene never closes cloned
IndexInputs, it will only do this on the original one.
The original instance must take care that cloned instances throw
AlreadyClosedException when the original one is closed.